Optically aligned hybrid semiconductor device and method

ABSTRACT

Two semiconductor chips are optically aligned to form a hybrid semiconductor device. Both chips have optical waveguides and alignment surface positioned at precisely-defined complementary vertical offsets from optical axes of the corresponding waveguides, so that the waveguides are vertically aligned when one of the chips is placed atop the other with their alignment surface abutting each other. The position of the at least one of the alignment surface in a layer stack of its chip is precisely defined by epitaxy. The chips are bonded at offset bonding pads with the alignment surfaces abutting in the absence of bonding material therebetween.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Patent Application No.62/141,650, filed Apr. 1, 2015, which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention generally relates to photonic integrated circuits,and more particularly relates to photonic integrated circuits formed oftwo or more optically aligned photonic chips and methods for fabricationthereof.

BACKGROUND

Photonic integrated circuits (PICs) may be formed in a single chip ormay include two or more optical chips when it is difficult orinconvenient to form all desired optical components of the circuit in asame material system. For example Silicon on Insulator (SOI) technologyenables to form optical circuits including a variety of passive andactive optical elements, such as optical waveguides, splitters,combiners, filters, wavelength multiplexers and demultiplexer,modulators, resonators, and the like, in a single SOI chip. Howeversilicon (Si) substrates are poorly suited for fabricating light sourcessuch as laser or light emitting diodes, which typically require compoundsemiconductor systems such as Gallium Arsenide (GaAs) or IndiumPhosphide (InP) based. Thus a PIC that includes both light-generatingelements and light processing elements may typically be a hybrid circuitwherein a laser or LED chip is optically and mechanically coupled to aSOI chip or the like containing light processing circuitry.

Multi-chip PICs require a continuous optical path between the chips.Optical waveguides of two optically adjacent chips in a multi-chip PICmust be aligned at a chip interface so that the optical loss isminimized and the integrity and quality of the optical signal can bepreserved. That typically requires a high precision three-axismechanical manipulation of the chips. The position of one chip withrespect to another may be described using an (x,y,z) Cartesiancoordinate system wherein the x- and y-axes are typically assumed to bedirected in the plane of the substrate of one of the chips, which may bereferred to as the carrier, and the z-axis is directed in a directionnormal to the main plane of the carrier substrate, which may also bereferred to as the vertical direction.

The fabrication and packaging of very small dimension waveguidestructures, often based on two or more material systems and differentsemiconductor chips, must be accomplished with high precision along anoptical reference line that requires precision alignment linear alongthe three axes and with respect to three angles of rotation about theseaxes. The final alignment must be performed in a low cost manufacturablemanner, then be fixed so that the alignment does not change withenvironment conditions and over the lifetime of the module orsubassembly. These linear and rotational alignments occur during theassembly process where a pair of chips or a multiplicity of chips arealigned to each other, and then assembled either to each other, orassembled to an additional chip or substrate, maintaining theiralignment to each other.

Modern semiconductor precision placement equipment is typically muchbetter suited for accurate micron- and sub-micron alignment of chips inthe (x,y) plane of the substrate, than in the z-direction that isvertical to the substrate plane. While accurate sub-micron alignments ontwo of the three axes, x and y, is normally within the rapid assemblytimes and capabilities of modern semiconductor precision positioningequipment, the end-to end alignment of two planar waveguides in thevertical (z) direction is typically more difficult due to the lack ofz-axis fiduciaries and alignment tools. The vertical alignment for theend-to-end optical transfer may also be very sensitive to tolerancebuildups that may occur when assembling multiple chips.

In order to overcome these difficulties and accommodate the variation inthickness and tolerance buildup in a direction orthogonal to thewaveguide's broad flat surface, active alignment techniques may beemployed, which include launching light into an optical waveguide of oneof the chips and measuring light transferred into the second chip whileadjusting the relative positioning and orientation of the chips so as tomaximize the light transfer between the chips. Although the activealignment techniques, when used, may provide desired accuracy of opticalalignment between chips, it may be inconvenient to use, and generallyincreases the cost, time and complexity of assembling multi-chip modulesand subassemblies.

There is a need for improved methods of optical alignment ofsemiconductor chips in multi-chip optical devices and circuits, and foroptically aligned multi-chip photonic integrated circuit devices thatare easier to assemble and align.

SUMMARY

Accordingly, an aspect of the present disclosure is directed to a hybridoptically aligned semiconductor device integrating two or moresemiconductor chips and a method of fabricating thereof wherein thesemiconductor chips are optically aligned in at least one directionusing one or more alignment surfaces serving as vertical alignmentstops. The semiconductor chips with alignment surfaces abutting eachother may then be bonded together using bonding pads that are verticallyoffset from the alignment surfaces. The alignment surfaces may beepitaxially-defined and abutting each other in the assembled hybriddevice in the absence of bonding material therebetween.

An aspect of the present disclosure provides a method of fabricating anoptically aligned hybrid semiconductor device, the method comprising:

a) providing a first semiconductor chip comprising a first alignmentsurface a and a first optical waveguide, wherein the first alignmentsurface is positioned with a first offset from an optical axis of thefirst optical waveguide;

b) providing a second semiconductor chip comprising a second alignmentsurface and a second optical waveguide wherein the second alignmentsurface is positioned with a second offset from an optical axis of thesecond optical waveguide wherein one of the first and second offsets isepitaxially defined in the respective first or second semiconductor chipto complement the other of the first and second offsets, so that whenthe first and second alignment surfaces are in contact abutting eachother the first and second waveguides are aligned in a direction normalto the first and second alignment surfaces;

c) positioning first and second semiconductor chips with the first andsecond alignment surfaces facing each other; and,

d) bringing the first and second semiconductor chips together until thefirst and second alignment surfaces come to a hard stop against eachother, with the first and second alignment surfaces being in directcontact abutting each other and the first and second optical waveguidesbeing optically coupled.

One aspect of the present disclosure provides an optically alignedhybrid semiconductor device comprising: a first semiconductor chipcomprising a first alignment surface and a first optical waveguidehaving an optical axis, wherein the first alignment surface ispositioned with a first offset from the optical axis of the firstoptical waveguide; and, a second semiconductor chip comprising a secondalignment surface and a second optical waveguide having an optical axis,wherein the second alignment surface is positioned with a second offsetfrom the optical axis of the second optical waveguide, wherein thesecond offset is complementary to the first offset; wherein at least oneof the first and second alignment surfaces comprises an epitaxialsurface; and, wherein the second semiconductor chip is disposed upon thefirst semiconductor chip so that the second alignment surface directlyabuts the first alignment surface in a contact therewith, and the firstand second optical waveguides are aligned and optically coupled.

One aspect of the present disclosure provides a method of fabricating anoptically aligned hybrid semiconductor device, the method comprising:

a) obtaining a first semiconductor chip comprising a first alignmentsurface and a first optical waveguide, wherein the first alignmentsurface is positioned with a first offset from an optical axis of thefirst optical waveguide;

b) obtaining a second semiconductor chip comprising a second alignmentsurface and a second optical waveguide, wherein the second alignmentsurface is positioned with a second offset from an optical axis of thesecond optical waveguide, wherein the first and second offsets arecomplementary so that when the first and second alignment surfaces arein contact with each other, the first and second waveguides extend in acommon plane;

wherein obtaining at least one of the first and second semiconductorchips includes using layer thickness-controlled epitaxy to grow a firststack of epitaxial layers so as to define a position of one of the firstand second alignment surfaces in the first stack of epitaxial layers ata layer interface between two of the epitaxial layers or at a layersurface of a top epitaxial layer in the first stack;

c) aligning first and second semiconductor chips with the first andsecond alignment surfaces facing each other; and,

d) bringing the first and second semiconductor chips together until thefirst and second alignment surfaces come to a stop against each other,with the first and second alignment surfaces being in direct contactabutting each other and the first and second optical waveguides beingoptically coupled.

According to an aspect of the present disclosure, the method may includegrowing the first stack of epitaxial layers that comprises a firstwaveguiding layer so as to cause the layer interface to be offset froman optical plane of the waveguiding layer by one of the first and secondoffsets. The method may further include selectively etching the firststack of epitaxial layers using a layer-selective etch so as to exposeat least a portion of a surface of one of the epitaxial layers at theepitaxial layer interface, and using the exposed surface of saidepitaxial layer as one of the first and second alignment surfaces.

In one aspect obtaining at least one of the first and secondsemiconductor chips may further include using thickness-controlledepitaxy to grow a second stack of epitaxial layers comprising a bufferlayer disposed over a second waveguiding layer so as to cause the topsurfaces of the buffer layer to be offset from an optical plane of thesecond waveguiding layer by the other one of the first or secondoffsets.

In accordance with an aspect of the present disclosure, the method mayinclude providing each of the first and second semiconductor chips witha bonding pad, the bonding pads positioned so as to face each other witha gap therebetween when the first and second semiconductor chips arebrought together in step (d) with the first and second alignmentsurfaces in direct contact abutting each other. The method may furtherinclude depositing a compliant bonding agent upon at least one of thebonding pads to a total height exceeding the gap. Step (d) may thencomprise bringing the first and second semiconductor chips together tofirst cause the compliant electrically conducting bonding agent to touchthe opposing bonding pad, and then bringing the first and secondsemiconductor chips further together until the first and secondalignment surfaces come to a stop against each other while forcing thecompliant bonding agent to partially expand sideways into a spacebetween the first and second semiconductor chips. The method may furtherinclude causing the compliant bonding agent to solidify, therebyproviding mechanical bonding between the first and second semiconductorchips. In some instances the bonding pads and the compliant bondingagent may be electrically conducting, and the bonding pads may bedisposed on electrically conducting surfaces of the respective first orsecond semiconductor chips, so as to provide electrical connectionbetween the first and second semiconductor chips in the opticallyaligned hybrid semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein will be described in greater detail withreference to the accompanying drawings, which are not to scale and inwhich like elements are indicated with like reference numerals, andwherein:

FIG. 1 is a schematic diagram representing a perspective view of ahybrid semiconductor device composed of two semiconductor chips withoptically aligned waveguides;

FIG. 2 is a schematic diagram illustrating a partial cross-section viewof a first semiconductor chip showing an optical waveguide and arecessed alignment surface below the waveguide;

FIG. 3 is a schematic diagram illustrating a cross-section view of asecond semiconductor chip showing an optical waveguide and an alignmentsurface above the waveguide;

FIG. 4 is a schematic diagram illustrating a hybrid device composed ofthe first and second semiconductor chips with the alignment surfaces incontact resulting in optically aligned waveguides;

FIG. 5 is a schematic diagram illustrating a top plan view of an exampleembodiment of the first semiconductor chip showing the optical waveguideand a recessed alignment surface below the waveguide with furtherrecessed contact pads;

FIG. 6 is a schematic diagram representing a cross-sectional viewthrough the recessed contact pads of the first semiconductor chip ofFIG. 5;

FIG. 7 is a schematic diagram illustrating a cross-section view of asecond semiconductor chip with an optical waveguide, an alignmentsurface above the waveguide, and contact pads above the alignmentsurface;

FIG. 8 is a schematic diagram of the first and second chips shown inFIGS. 6 and 7 in cross-sectional view during assembly, with the chips invery close proximity to each other, in a state of y-axis alignment,illustrating how salient features on each chip align;

FIG. 9 is a schematic diagram representing a cross-sectional view of theassembled hybrid device with the first and second chips shown in FIGS. 6and 7 optically aligned;

FIG. 10A is a schematic diagram representing a cross-sectional view ofthe first semiconductor chip of FIG. 5, with bonding agent disposed onthe recessed contact pads prior to the assembly;

FIG. 10B is a schematic diagram representing a step in the assembly ofthe first and second semiconductor chips according to an embodiment ofthe assembly method;

FIG. 11A is a schematic cross-sectional view of a first semiconductorchip with an etch-stop layer and metal pads over it prior to etching toexpose an alignment surface;

FIG. 11B is a schematic cross-sectional view of the first semiconductorchip with an etch-stop layer and an exposed alignment surface;

FIG. 12A is a schematic diagram illustrating the placement of a secondsemiconductor chip with matching contact pads into a recess in the firstsemiconductor chip;

FIG. 12B is a schematic diagram of an assembled hybrid semiconductordevice formed of optically aligned first and second semiconductor chipsof FIG. 12A;

FIG. 13A is a schematic cross-sectional view of a first semiconductorchip with an etch-stop layer above recessed metal pads prior to etchingto expose an alignment surface;

FIG. 13B is a schematic cross-sectional view of the first semiconductorchip of FIG. 13A after a first etch to expose the alignment surface;

FIG. 13C is a schematic cross-sectional view of the first semiconductorchip of FIG. 13B after a second etch to expose the contact pads;

FIG. 14 is a schematic cross-sectional view of an embodiment of anassembled hybrid semiconductor device formed of the first semiconductorchip of FIG. 13C and a second semiconductor chip that is opticallyaligned with the first semiconductor chip and bonded thereto at matedcontact pads;

FIG. 15 is schematic cross-sectional view of an assembled hybridsemiconductor device formed of two semiconductor chip that are opticallyaligned using epitaxially-defined vertical-alignment stops and bondingpads located away from alignment surfaces;

FIG. 16A is a schematic diagram showing a vertical cross-section of anassembled hybrid semiconductor device with alignment surfaces defined bypillars in a second chip and trenches in a first chip;

FIG. 16B is a schematic diagram showing a vertical cross-section of anassembled hybrid semiconductor device with alignment surfaces defined bypillars in a first chip and trenches in a second chip;

FIG. 16C is a schematic diagram showing a vertical cross-section of anassembled hybrid semiconductor device with alignment surfaces defined bypillars in both first and second chips;

FIG. 17A is a schematic cross-sectional view of a semiconductor chipwith an alignment surface provided by a buffer layer grown over awaveguiding layer stack;

FIG. 17B is a schematic cross-sectional view of a semiconductor chipwith an alignment surface provided by a pillars defined in a bufferlayer grown over a waveguiding layer stack;

FIG. 18A is a schematic cross-sectional view of an InP-based laser diodechip with a buried heterostructure (BH) waveguide;

FIG. 18B is a schematic cross-sectional view of an InP-based laser diodechip with a BH waveguide and an alignment surface provided by a bufferlayer grown over the BH layer structure;

FIG. 18C is a schematic cross-sectional view of an InP-based laser diodechip with a BH waveguide and an alignment surface provided by pillarsdefined in a buffer layer grown over the BH layer structure;

FIG. 19A is a schematic cross-sectional view of a second chip with aridge waveguide defined in a waveguiding layer and electrical contactsconfigured for electrically biasing the ridge waveguide;

FIG. 19B is a schematic cross-sectional view of the second chip of FIG.19A with the upper cladding and the waveguiding layer selectivelyremoved at designated locations to form one or more alignment surfaces;

FIG. 20 is a schematic perspective view illustrating the placement of ahermetic lid over a second semiconductor chip disposed in a recess in afirst semiconductor chip;

FIG. 21 is a schematic diagram showing a cross-section of a hybridsemiconductor device with a second semiconductor chip disposed in arecess in a first semiconductor chip and covered with a sealed lid;

FIG. 22 is a schematic sectional side view of an example hybridsemiconductor device with two semiconductor chips optically aligned on acommon carrier;

FIG. 23 is a schematic diagram of a gain chip and a SiP chip opticallyaligned on a common carrier and coupled to a fiber array in a side view(a) and a plan view (b).

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular opticalcircuits, circuit components, techniques, etc. in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well-known methods,devices, and circuits are omitted so as not to obscure the descriptionof the present invention. All statements herein reciting principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

Note that as used herein, the terms “first,” “second” and so forth arenot intended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. The terms“obtaining” or “providing,” when used herein with relation to a step oroperation of a method or a process, are intended to mean making anelement, object, or feature available for use in the method or process,and encompass designing, either fully or in part, fabricating, eitherfully or in part, or otherwise acquiring the object, element, orfeature, including for example purchasing or obtaining from an externalsupplier. The terms ‘photonic integrated circuit,’ or PIC, and‘integrated lightwave circuit’ may be used herein interchangeably. Theterm “optical waveguide” is used herein to refer to any optical elementor structure that provides optical confinement in at least one dimensionand wherein light of a target wavelength or wavelengths can propagate.

The term “planar optical waveguide” may refer to any optical waveguidethat is provided on a substantially planar substrate, carrier, or layer.Examples of such waveguides include ridge waveguides, buried waveguides,semiconductor waveguides, silica-based waveguides, polymer waveguides,other high-index or low-index waveguides, and many other examples notexplicitly set forth herein but nevertheless falling within the scope ofinventive concepts disclosed and/or claimed herein. Examples of suitablesubstrates include, but are not limited to semiconductor substrates,crystalline substrates, silica or silica-based substrates, substratesbased on other glasses, ceramic, metal, and others not explicitly setforth herein but nevertheless falling within the scope of inventiveconcepts disclosed and/or claimed herein. The term “vertical” is usedherein in relation to a plane of an optical chip or a carrier and isusually the direction of the epitaxial growth of the chip; verticaldirection means a direction that is perpendicular to a main face of theoptical chip at which an optical waveguide is defined, and perpendicularto the plane of the optical waveguide. The vertical direction may alsobe referred to herein as the transverse direction. In embodiments havingan alignment surface configured to work as an alignment stop when twochips are brought together during assembly, the vertical direction maybe normal to the alignment surface. Structural elements of the opticalchip may be described with reference to a Cartesian coordinate system(x,y,z) associated with the chip, wherein the z axis is directed in thevertical direction normally to the main face of the chip and along thedirection of the epitaxial growth. The terms “vertically offset” and“vertically displaced” are used herein interchangeably to mean beingdistanced in a direction that is perpendicular to the alignment surface,or to the plane of the optical waveguide, i.e., the z-axis direction.The term “offset” as a noun is used herein to refer to vertical offsetbetween two elements or layers of a chip, i.e., a displacement of one ofthe elements from the other along, or in projection on, the verticaldirection normal to the layers of the chip. An offset may be positive ornegative, depending on the order of the respective elements along az-axis that is normal to the plane of the chip. The positive directionof z axis may be, for example, the direction from an alignment surfaceaway from the chip. The term “epitaxially defined” is used herein tomean defined by and during the epitaxial growth of the chip layers. Theterms “epitaxially defined offset” and “epitaxially defineddisplacement” are used herein to refer to a distance or offset betweenepitaxially defined features of a stack of semiconductor or dielectriclayers that are grown using a layer thickness-controlled epitaxy, in adirection generally normal to the layers, for example in the directionof epitaxial growth. It may refer to an offset between an optical axisor plane of a waveguiding layer and a top or bottom surface of anotherepitaxial layer of the stack. The location of an optical plane or axisof a waveguiding layer in the layer stack may be accurately determinedfrom known thicknesses and refractive indexes of the waveguiding layerand its cladding layers as known in the art. The optical plane of awaveguiding layer is understood herein as a plane within the waveguidinglayer wherein the vertical profile of a main optical mode supported bythe waveguiding layer has maximum; for a layer stack that issubstantially optically symmetric with respect to the waveguiding layer,the optical plane is the median plane of the waveguiding layer, i.e.,the plane in the waveguiding layer that divides the waveguiding layer intwo vertically-stacked sub-layers of equal height. In some embodiments,the optical plane may be assumed to be the median plane of thewaveguiding layer. An optical axis of a waveguide is understood to meana line traced by a maximum intensity point of a main optical mode of thewaveguide when both lateral and vertical confinement of the mode aretaken into account. In a transversely symmetric or nearly-symmetricwaveguide it corresponds to a central line thereof. In some embodiments,the optical axis may be assumed to be the centerline of the waveguidecore. An optical plane of a waveguide is a plane comprising the opticalaxis of the waveguide and parallel to the plane of the chip and to thesubstrate. Two chips with optical waveguides defined therein are said tobe optically aligned when their respective optical waveguides aremutually aligned so as to be optically coupled through their end-faces.

Example embodiments disclosed herein relate to fabrication of hybridoptical devices comprised of two or more optically aligned semiconductorchips, each of said chips fabricated by forming a stack of layers ofdifferent materials upon a planar substrate, and having at least oneplanar optical waveguide defined in it, for example using knownlithographic techniques. Portions of the layer structure and/or layerthicknesses of at least one of the chips are epitaxially defined duringthe chip fabrication so that when one of the chips is placed upon theother in a specified manner, or both are placed on a same carrier in aspecified manner, optical waveguides are vertically aligned so as toensure efficient optical coupling between the waveguides.

In various embodiments disclosed herein the chips to be opticallyaligned may be bonded to one another, or to a third device or carrier,by a bondable material such as but not limited to a solder alloy or anepoxy.

In various example embodiments disclosed herein each of the chipsincludes an optical waveguide and an alignment surface that isconfigured to assist in the vertical alignment of the chips, so thatwhen one of the chips is placed upon the other with the alignmentsurfaces in contact abutting each other, optical planes of the two chipsare coincidental and optical axes of the waveguides of the two chips liein the same (x,y) plane so that the optical waveguides can be madeoptically aligned by adjusting the position of one of the chips in the(x,y) plane without requiring any further adjustment in the vertical, ortransverse, direction. Such alignment surfaces may also be referred toherein as vertical-alignment surfaces, alignment-stop surfaces, or asalignment stops. The vertical offset between the waveguide's opticalaxis and the alignment surface of at least one of the chips ispreferably epitaxially defined, i.e., defined by means of, and during, acontrolled epitaxial growth of one or more layers of the chip. Thealignment surface may be, for example, a top surface or a bottom surfaceof an epitaxial layer, which may be termed alignment layer. Sinceepitaxial growth can be tightly controlled using techniques that arewell-known in semiconductor manufacturing, the vertical displacement ofoffset of the epitaxially-defined alignment surface with respect to theoptical axis of the optical waveguide formed in the same chip may beprecisely controlled with an accuracy of better than +\−200 nm, and inmany cases with accuracy about or better than +\−100 nm. In at leastsome embodiments, the semiconductor chips are fabricated so that thevertical offsets between the alignment surfaces and the optical axes oftwo chips to be aligned are equal in value with accuracy of at least+\−200 nm, or preferably with the accuracy within +\−100 nm. However thedirection of the alignment surface offset may differ between the twochips when one of the chips is to be flip-chip mounted on top of theother. Offsets that are equal in value but different in direction orsign may be referred to herein as complementary.

In various embodiments disclosed herein the chips may have chemicallystable, non-compressible, solid alignment surfaces engineered to belocated at a known vertical offset from the optical axis of the chip. Insome embodiments, the alignment surfaces are planar; in otherembodiments they may be at least partially non-planar and configured tomate with each other. Preferably, the chemically stable alignmentsurfaces are non-reactive relative to the objects they contact in theassembled hybrid device. Preferably, the chemically stable alignmentsurfaces are surfaces of epitaxially grown semiconductor or dielectriclayers and are absent of metal or any other material deposited by anon-epitaxial technique in which the thickness of the deposited materialcannot be controlled with the desired accuracy. In some embodiments thealignment surface or surfaces may be provided by a surface of a layerthat is formed using a layer growth technique that is non-epitaxial, butnevertheless allows for a layer thickness control with the desiredaccuracy, preferably so as to define relevant offsets in the structurewithin +\−200 nm from a target value, and more preferably within +\−100nm.

Referring to FIG. 1, there is illustrated a perspective view of arepresentative hybrid semiconductor device 100 formed of a firstsemiconductor chip 101 and a second semiconductor chip 102, which may bereferred to hereinafter simply as the first chip 101 and the second chip102, respectively. The second semiconductor chip 102 is placed on top ofthe first semiconductor chip 101 and bonded thereto in an opticallyaligned fashion. The first chip 101 may be, for example, silicon-based,such as a SOI PIC chip, and the second chip 102 may be, for example, anInP or GaAs based chip. A silicon (Si) based optical chip may also bereferred to herein as a SiP (silicon photonic) chip. The first chip 101may also be referred to herein as the carrier chip. Also illustrated isa reference Cartesian coordinate system associated with the first chip101 and having axes X, Y, and Z that may be used in the description. Thechips 101 and 102 include optical waveguides formed therein which areoptically aligned in the assembled hybrid device, and may furtherinclude electrical interconnects. Each of the chips 101, 102 alsoincludes an alignment surface, with the alignment surface of at leastone of the chips preferably being in the form of, or including, anepitaxial surface, i.e. a layer surface that is formed by epitaxy.

FIG. 2 illustrates a schematic partial cross-section view of the firstchip 101 in an example embodiment thereof, with the cross-section takenalong the x-axis. The first chip 101 includes a waveguiding layer 121forming a core of a first optical waveguide, such as the opticalwaveguide 221 indicated in FIG. 5, and a first vertical-alignmentsurface 151 that is positioned with a first vertical offset d₁ from anoptical axis or plane 141 of the waveguiding layer 121, which is alsoreferred to herein as the first offset 151. The first vertical-alignmentsurface 151 is useful for passive vertical alignment of the chips 101,102 against each other as described hereinbelow, and may also bereferred to herein as the first alignment-stop surface 151 or simply asthe first alignment surface 151. In the illustrated embodiment the firstalignment surface 151 and the first optical waveguide may be defined byan epitaxial growth of a stack of layers 171, 111, 121, and 131 on asubstrate 161. The waveguiding layer 121 is sandwiched between claddinglayers 111 and 131 of a lower refractive index. The waveguiding layer121 defines the vertical position of the optical waveguide 221, anexample of which is illustrated in FIGS. 5, 6, and as such defines thevertical position thereof in the chip. In a representative embodiment,the waveguiding layer 121 may have a thickness in the range of 300-800nm, for example about 500 nm. The first alignment surface 151 may be anexposed portion of a top epitaxial surface of the alignment or bufferlayer 171 wherein the cladding and waveguiding layers 111, 121, and 131are removed by layer-selective etch to form a recess 120, or not grown.The exact vertical position of an optical axis 141 of the waveguiderelative to the first alignment surface 151 depends on the refractiveindex profile of the epitaxial layer structure 111/121/131 in thevertical (z-axis) direction and may be accurately set by a controlledepitaxial growth as known in the art. The high layer-thickness accuracyafforded by the epitaxial growth enables to control the vertical offsetd₁ with a high precision, typically with an accuracy of at least +\−200nm or preferably within +\−100 nm.

The relative vertical positions of various elements of a chip may bedescribed with reference to a Cartesian coordinate system associatedwith the chip, in which the z-axis is perpendicular to the plane of thechip and/or to the alignment surface and is directed away from the chip.The z-axis datum may, for example, be at the alignment surface. In FIG.2 such coordinate system associated with the first chip 101 is indicatedat 191; in the coordinate system 191 the vertical offset d₁ is positive,indicating that the waveguiding layer 121, and the corresponding opticalwaveguide 221, is positioned above the alignment surface 151 relative tothe first chip.

FIG. 3 illustrates a schematic cross-sectional view of the second chip102 in an example embodiment thereof. It includes a waveguiding layer122 and a second vertical alignment surface 152 that is positioned witha second vertical offset d₂ from an optical axis or plane 142 of thewaveguiding layer 122. The second vertical offset d₂ may also bereferred to herein as the second offset d₂. In a representativeembodiment, the waveguiding layer 122 may have a thickness in the rangeof 300-800 nm, for example about 500 nm. The second vertical-alignmentsurface 152, which will also be referred to herein simply as the secondalignment surface 152, is useful for passive vertical alignment of thechips 101 and 102 when assembling the hybrid device 100 as describedhereinbelow. In one embodiment, the second alignment surface 152 and thewaveguiding layer 122 may be formed by growing a stack of layers 112,122, and 132 on a substrate 162 using layer thickness controlled growthtechnique such as epitaxy, with the layers 112 and 132 defining the topand bottom claddings. The waveguiding layer 122 is sandwiched betweencladding layers 112 and 132 of a lower refractive index, defining thevertical position of a core of an optical waveguide, such as an opticalwaveguide 222 illustrated in FIGS. 7-9. The second alignment surface 152may be an exposed portion of a top surface of the top cladding layer132, which serves in this case as an alignment layer. The verticalposition of an optical axis 142 of the waveguide relative to thealignment surface 152 depends on the refractive index profile of theepitaxial layer structure 112/122/132 in the vertical, or z-axis,direction and may be accurately determined as known in the art fromknown layer thicknesses and refractive indexes. The high layer thicknessaccuracy afforded by the epitaxial growth enables to control thevertical offset d₂ with a high precision, typically about or better than+\−200 nm, or preferably within +\−100 nm.

Note that the second alignment surface 152 is vertically offset from theoptical axis or plane 142 of the corresponding waveguide or waveguidinglayer 122 in an opposite direction to the displacement of the alignmentsurface 151 relative to the waveguiding layer 121 in the first chip 101;while the first alignment surface 151 is positioned above thewaveguiding layer 121 of the first chip 101, the second alignmentsurface 152 is positioned below the waveguiding layer 122 of the secondchip 102. Here, the terms ‘above’ and ‘below’ refer to the face of thechip where the respective alignment surface is defined. In a coordinatesystem 192 associated with chip 102 wherein the z-axis is directednormally to the second alignment surface 152 away from the chip, thesecond vertical offset d₂ is negative.

With reference to FIG. 4, in a preferred embodiment the first and secondvertical offsets d₁ and d₂, at least one of which is epitaxially definedwith a high precision during the fabrication of chips 101 and 102, areselected to be substantially equal in value, so that when chip 102 isplaced on top of chip 101 with its alignment surface 152 facing thealignment surface 152 of the first chip 101 in a firm contact therewith,the optical waveguiding layers 121 and 122 are in a precise verticalalignment with each other, and extend in a common plane, which isindicated in FIG. 4 at 255 and may be generally parallel to thealignment surfaces. Offsets that are opposite in direction or sign butare equal in value, or have values that optimize optical couplingbetween the waveguides, such as the vertical offsets d₁, d₂ illustratedin FIGS. 2 and 3 and described hereinabove, may be referred to herein ascomplementary. Here “opposite in a direction or sign” means oppositerelative to a datum feature of the chip for which the respective offsetis defined. Once the chips 101 and 102 are brought together so thattheir alignment surfaces 151, 152 are in contact abutting each other,the relative position of the chips may be further adjusted as desired inthe (x,y) plan so that the first and second optical waveguides areoptically coupled with a desired coupling efficiency.

Thus, in one embodiment the method of fabricating an optically alignedhybrid device 100 may include the following general steps:

A) obtaining a first semiconductor chip, such as for example chip 101illustrated in FIG. 2, with a first alignment surface 151 and a firstoptical waveguide defined therein, wherein the first alignment surface151 is positioned with a first offset d1 from an optical axis of thefirst optical waveguide;

B) obtaining a second semiconductor chip, such as for example chip 101illustrated in FIG. 2, with a second alignment surface 152 and a secondoptical waveguide defined therein, wherein the second alignment surface152 is positioned with a second offset d2 from an optical axis of thesecond optical waveguide, wherein the first and second offsets d1, d2are complementary so that when the first and second alignment surfaces151, 152 are in contact with each other, the first and second waveguidesextend in a common plane;

C) bringing the first and second semiconductor chips 101, 102 togetheruntil the first and second alignment surfaces 151, 152 come to a stopagainst each other, and remain in a direct contact abutting each otherwith the first and second optical waveguides being optically coupled inthe assembled hybrid device.

In one embodiment, fabricating at least one of the first semiconductorchip 101 and second semiconductor chip 102 includes usingthickness-controlled epitaxy to grow a first stack of epitaxial layers,such as the layer stack 171/111/112/131 of FIG. 2 or the layer stack112/122/132 of FIG. 3 so as to define a position of one of the first andsecond alignment surfaces 151, 152 in the first stack of epitaxiallayers at a layer interface between two of the epitaxial layers, such asthe layer interface between layers 171 and 111 in FIG. 2, or at a layersurface of a top epitaxial layer in the first stack, such as the toplayer surface of the top cladding epitaxial layer 132 in FIG. 3.

FIGS. 5 and 6 schematically illustrate a top plan view of an exampleembodiment of the first chip 101 in the absence of the second chip 102,and a plan sectional view of the first chip 101 taken along the line AA,respectively. FIG. 3 may be viewed as the partial cross-section B-B ofthe chip 101 as illustrated in FIG. 5. The optical waveguide 221 extendsalong the x-axis, and terminates at a recess or cavity 120 defined atthe top face 71 of the first chip 101. The top face 71 of the first chip101 may also be referred to as the main face of the chip, or as thechip-mounting face. A first alignment surface 151 at the bottom of thecavity or recess 120 is displaced from an optical axis of the waveguide221 in the vertical direction (z-axis) by the first vertical offset d₁.The exact value of the first vertical offset may be epitaxially definedas described hereinabove. Bonding pad or pads 181 are in turn verticallyoffset from the first alignment surface 151 further away from theoptical waveguide 221; in the shown embodiment they are disposed inrecesses at the bottom of the cavity 120. The bonding pad or pads 181are configured to adhere to a bonding agent for bonding to the secondchip 102. In one embodiment, the bonding pad or pads 181 may be made ofan electrically conducting material such as metal. In one embodiment,the bonding pad or pads 181 may serve as electrical contacts and may bereferred to as electrode pads or as electrical contact pads.

FIG. 7 schematically illustrates a cross-sectional view of an exampleembodiment of the second chip 102 prior to mounting on the first chip101 of FIGS. 5, 6. The second chip 102 includes a second opticalwaveguide 222 that terminates at an edge of the chip. A second alignmentsurface 152 is vertically displaced from the optical axis or plane ofthe second optical waveguide 222 by a second offset d₂. In oneembodiment the second offset d₂ was epitaxially defined during theepitaxial growth of the second chip 102 to be substantially equal invalue to the first vertical offset d₁ of the first chip 101, preferablywith the accuracy of +\−200 nm or better, or more preferably with theaccuracy of +\−100 nm or better. Electrical contact or bonding pads 182may further be provided, for example leveled with the second alignmentsurface 152 or with a vertical offset therefrom. The bonding pads 182may be disposed to mate with the bonding pads 181 of the first chip 101,as shown in FIGS. 8 and 9 and described hereinbelow.

With reference to FIGS. 8 and 9, the second chip 102 may be flip-chipmounted in the recess formed in the main face of the first chip 101. Theprocess of assembly of the optically aligned hybrid device may includealigning first and second semiconductor chips 101, 102 with the firstand second alignment surfaces 151, 152 facing each other as illustratedin FIG. 8, and bringing the first and second semiconductor chips 101,102 together until the first and second alignment surfaces 151 and 152come to a stop against each other, with the alignment surface 151abutting the alignment surface 152 as illustrated in FIG. 6. Since atleast one and preferably both of the first and second distances d₁ andd₂ is epitaxially defined to match the other with a high degree ofaccuracy, in a relative chips position where the first and secondalignment surfaces 151, 152 are parallel and in direct intimate contactwith each other, the first and second optical waveguides 221, 222 arealigned in the vertical, i.e. z-axis, direction, so that their opticalplanes merge.

Advantageously, the precise vertical alignment of the chips may beaccomplished passively without the need to monitor chip-to-chip opticaltransmission. During assembly, automated precision placing equipmentused in this process may hold chip 102 in proper alignment positionalong the X and Y axes to chip 101 and in close proximity along the zaxis, and then may move chip 102 along the Z axis to contact chip 101.In some embodiments, the passive alignment of the chips may be assistedby camera imaging. In some embodiments, the chips may be furtheroptically alignment in the (x,y) plane using active alignment assistedby an optical feedback. In some embodiments, the alignment may beassisted by utilizing surface tension of a liquid bonding agent, such asmelted solder, to move one of the chips in a position of opticalalignment with the other chip.

In the illustrated embodiment the first electrical contact or bondingpad 181 is vertically displaced relative to the first alignment surface151 in the z-axis direction away from the first optical waveguide 121,so that when the first and second alignment surfaces 151, 152 arebrought in contact with each other there exists a gap G 177 between thefirst and second electrical contact or bonding pads 181 and 182. Thiscontact gap may be filled with a bonding material 188, such as acompliant electrically conducting bonding agent, for example a solder ormetal-filled epoxy, to bond the chips together and/or to provide anelectrical connection between the pads 181 and 182. In one embodiment,the bonding pads 181 and 182 may be form of electrically conductingmaterial, for example metal or metal alloy. In one embodiment,electrically conducting bonding pads 181 and 182 may be disposed uponsurfaces of the first and second chips 101, 102 which are alsoelectrically conducting, for example metallized or formed of a suitablydoped semiconductor material, so as to provide electrical connectionsfrom the bonding pads to other areas of the respective chips.

In one embodiment, prior to bringing the chips together, the bondingagent 188, such as solder, may be disposed on one or both of the contactpads 181, 182 in an amount suitable for forming the electricalconnection therebetween when the alignment surfaces 151 and 152 are incontact abutting each other. In some embodiments, it is sufficient ifthe bondable material 188 is present on one of the two chips that are tobe aligned and bonded, as illustrated in FIG. 10.

With reference to FIGS. 10A and 10B, in one embodiment the bondingmaterial 188 such as solder may be disposed on the bonding pad or padsto a total height that exceeds the contact gap G 177, and then madepliable during the assembly process to fill the gap 177, for example byheating. In this embodiment, as the two chips are brought into contactwith each other, the bondable material 188 makes first contact with anopposing bonding pad, or solder disposed on the opposing solder pad,before any alignment surface can touch another alignment surface as isillustrated in FIG. 9. The assembly process then heats or otherwiseprovides impetus for the bonding material 188 to soften and form aconductive, for example metallic, or non-conductive adhesive bondbetween the two chips. As the bonding material 188 softens, the firstand second semiconductor chips 101, 102 are brought further togetheruntil the first and second alignment surfaces 151, 152 come to a stopagainst each other while possibly forcing the bonding material 188 topartially expand sideways into a space between the first and secondsemiconductor chips.

In one embodiment the bonding agent 188 may be disposed upon one or bothof the bonding pads 181, 182 to a total height that is less than theelectrode gap G, but in the amount sufficient to fill the gap 177 so asto connect the bonding pads 181, 182 together. In this embodiment, thebonding material 188 may be made to gather up and fill the gap 177, forexample by heating, after the alignment surfaces 181, 182 are broughtinto contact, so as to connect together the bonding pads 181, 182. Inone embodiment, the bonding material 188 may be made to fill the gap 177by wicking. In some embodiments, the bonding material in a liquid phasemay be used to assist in the relative alignment of the chips by surfacetension.

In one embodiment the bonding material 188 on chip 101, for examplesolder alloy, is caused to bond to the bonding pad of chip 101, forexample by a rapid application of heat, pressure, and inert or reducingatmosphere. When the bonding material softens and melts and reflows ontothe electrodes, chip 102 may be brought into more intimate contact withchip 101, with the alignment surfaces 152 of chip 102 coming to finalrest upon the passive alignment surfaces 151 directly abutting eachother. In this state optical planes or axes of the chips also becomecoincident, resulting in waveguide optical alignment. The devices arethen cooled and the bonding material such as solder solidifies,permanently joining both chips to form a single compound hybrid device100 with aligned waveguides.

In general, the bonding material 188 can be present during the chipassembly on the bonding pad or pads of either chip, as may beconvenient. In embodiments where a plurality of chips are bonded one toanother, a given chip may carry the bonding material for one bondingstep, and may be lacking bonding material for a second bonding step if abonding material is present on the other chip used in the second bondingstep. In some embodiments, the bonding material is electricallyconductive, such as solder or metal-filled epoxy. In other embodiments,the bonding material may be non-conductive, such as epoxy or adhesive.

Notably, in the hybrid optical device 100 or 100 a the alignmentsurfaces 151 and 152 of the first and second chips 101, 102 function asvertical-alignment stops but are not used for boding; instead elementsthat are separate from the alignment surfaces 151, 152, namely thebonding pads 181, 182, are utilized to bond the chips together. Thisseparation of the alignment-stop functions and boding function may beadvantageous for accurate optical alignment of the chips, since thethickness of a bonding material between two surfaces being bonded in thefinal hybrid device is difficult to control with the sub-micron accuracydesired for the optical alignment between the waveguides. Furthermore,chemical reactions that may be associated with bonding, such as thereaction of solder to a metal of the bonding pad, may consume asignificant depth of the bonding pad, thereby rendering its surface nolonger at the target distance from its referenced optical plane or axis.A similar problem of surface deformation leading a to waveguidemisalignment may also be present when two chips are bonded usingtextured cold welds, which may be used to bond a metallized surfacehaving sharp projections to another metallized surface.

The alignment surfaces 151 and 152 that function as vertical-alignmentstops may be epitaxially-defined and provided by the exposed surface ofa top layer of a chip, or by a surface of one of the intermediateepitaxial layers of the chip, with the exact position of the alignmentsurface defined by the position of an epitaxial layer interface, thatmay be precisely controlled relative to the center of the waveguidinglayer by layer-thickness controlled epitaxy with an accuracy of at least+\−200 nm, or +\−100 nm. An intermediate epitaxial layer that serves asan alignment layer may be at least partially exposed by layer-selectiveetching so that its exposed surface may serve as the alignment surface.The material of the alignment layer should preferably be selected to behighly resistant to the selected etching process, so that the alignmentlayer may function as an etch-stop layer.

In example embodiments described hereinbelow one of the chips of thehybrid semiconductor device 100, for example the first chip 101, may besilicon-based and may be referred to hereinafter the silicon photonic(SiP) chip. In some embodiments, the SiP chip can be built on asilicon-on-insulator (SOI) wafer that includes a silicon substrate, aburied oxide layer (BOX) and a silicon device layer. The SiP chip caninclude a number of passive and active components including grating andedge couplers, waveguides, splitters, modulators and germaniumphotodetectors. There can be metal interconnects for electrical routingon the SiP chip. The SiP chip can be fabricated in a CMOS-style process.Edge couplers can be formed in the SiP chip using the silicon devicelayer that exist on the SOI wafer and/or using additional layers thatmay be deposited during the device fabrication and processing. Opticalwaveguides can be formed in various layers of the SiP chip. Some of thelayers may be planarized, for example using chemical-mechanicalpolishing (CMP). In some embodiments, the optical waveguide orwaveguides formed in the SiP chip may utilizes silicon dioxide layers ascladding layers. In various embodiments, the thickness of the claddinglayers can be finely tuned to ensure precise vertical, or Z-axisplacement of the waveguiding layers.

Referring to FIGS. 11A and 11B, there is illustrated a sidecross-sectional view of an example SiP 301 chip at different stages offabrication. The SiP chip 301 includes an optical waveguide 331 foredge-coupling to an optical fiber, an optical waveguide 330 foroptically coupling to a second chip, an etch-stop layer 320, and a metallayer patterned for forming electrical contact or bonding pads 310, allsupported by a silicon or SOI base substrate or handle 350. The SiP chip301 as illustrated in FIG. 11A may be fabricated using a sequence ofepitaxial or high-precision non-epitaxial growth steps, selectiveetching steps, selective metal deposition and re-growth. The fabricationprocess may include, for example, depositing a first cladding layer 315over a Si or SOI substrate 350, followed by deposition or epitaxialgrowth of the etch-stop layer 320, the second cladding layer 325 as athird layer, a first waveguiding layer at the location of the opticalwaveguide 330, a third cladding layer 335, and a second waveguidinglayer at the location of the optical waveguide 331. In some embodimentsthe optical waveguide 331 may be absent. The metal pads 310 may beformed upon the etch-stop layer 320 using known lithographic techniquesprior to the growth of the second cladding layer 325. The etch-stoplayer 320 may be used as the first alignment layer, with its top surfacedefining the vertical, or z-axis position relative to the waveguide 330of the first alignment surface that will be used as a vertical alignmentstop in a subsequent ship assembly. Accordingly, the deposition of thesecond cladding layer 325 and the first waveguiding layer defining thecore of the first optical waveguide 330 may be performed in a controlledmanner to ensure a desired value of the first vertical offset d₁separating the optical axis or plane 341 of the first optical waveguide330 from the top surface of the etch-stop layer 320. The layer structureindicated in FIG. 11A may be formed using wafer-scale fabrication.

The device structure illustrated in FIG. 11A may then be processed usinga suitable selective etch technique to remove all of the layers up tothe etch stop layer 320 in a portion of the chip, as defined for exampleby a hard mask 335. The selective etch step may result in a structureillustrated in FIG. 11B, wherein chip 301 now has a cavity or recess 350that opens at the top face of the chip, with an exposed top surface 351of the etch-stop layer 320 at the bottom of the cavity 350 forming thefirst alignment surface. Depending on the materials used, the selectiveetch technique may be, for example, a deep oxide etch or a suitable wetetch, with the material of the etch-stop layer 320 selected to beresilient to the etch.

By way of example, the cladding layers 315, 325 and 335 may be formed ofsilicon dioxide, the waveguiding layers 330, 331 may be formed ofsilicon, polysilicon, amorphous silicon, silicon dioxide, doped silicon,silicon nitride, germanium, or silicon germanium, while the etch stoplayer 320 may be a layer of silicon dioxide or silicon nitride, and thereactive ion etch (RIE) technique may be used to remove the cladding andwaveguiding layers in the selected portion of the chip.

Turning now to FIGS. 12A and 12B, an optically-aligned hybridsemiconductor device may be fabricated by placing a second chip 402, forexample an InP-based chip, into the cavity or recess 350 at the top ofthe first chip 301. The second chip 402 has a second alignment surface451, and in the process of assembly may be flipped to be oriented withthe second alignment surface 451 facing the first alignment surface 351of the first chip 301 and parallel thereto. A metal contact pad or pads410 may be disposed in recesses in the second alignment surface 451, tobe vertically offset therefrom and in a location suitable for matingwith the first contact pads 310 of the first chip 301 when theirrespective alignment surfaces 351, 451 abut each other. The second chip402 further includes an optical waveguiding layer 430 forming a core ofan optical waveguide which terminates at a side of the second chip andwhich will be referred to herein as the waveguide 430. The opticalwaveguiding layer or waveguide 430 is disposed so that its optical axisor plane is vertically offset from the alignment surface 451 by a seconddistance d₂, which is preferably epitaxially defined to be equal invalue to the first vertical offset d₁, as discussed hereinabove, with anaccuracy of at least +\−200 nm, and preferably +\−100 nm.

Accordingly, when the second chip 402 is placed upon the first chip 301with their alignment surfaces 451 and 351 abutting each other and in afirm contact across flat portions thereof, the optical waveguide 330 and430 of the chips are in a good optical alignment with each other, withtheir optical planes or optical axes substantially aligned andcoincidental, as illustrated in FIG. 12B. If required, precisionplacement equipment may be used to further align the chips in the (x,y)plane relative to each other so as to further improve optical alignmentand coupling between the chips' waveguides.

Referring further to FIG. 12B, in the illustrated embodiment of theassembled device 300 optical waveguides 330 and 430 are aligned in thevertical, i.e., z-axis, direction, and metal contacts 310 and 410 are inelectrical communication provided by a conductive bonding agent 440,such as solder, which bonds the chips together and provides electricalcontact between their metal bonding pads 310 and 410, thereby alsoproviding a thermal contact between the chips. In one embodiment anelectrical contact (not shown) for contacting an external circuit may beprovided at the two-chip device 300, on either the first chip 301 or thesecond chip 402. If provided on the first chip 301, this contact can bemade on the same metal layer as that used to contact the second chip402, or a different metal layer.

Referring now to FIGS. 13A-13C and FIG. 14, in another embodiment of theSiP chip that is generally indicated as chip 302, the etch-stop layer320 may be deposited over the metal contact pad 310, possibly with anintermediate planarizing dielectric or semiconductor layer therebetween,so that the contact pads are vertically offset further away from theoptical waveguide 330 than the top surface of the etch-stop alignmentlayer 320. As illustrated in FIG. 13A, the layer structure of the chip302 may be in other respects similar to the layer structure of the chip301 illustrated in FIG. 11A, with the etch stop layer 320 functioning asthe first alignment layer. In one embodiment the waveguiding layer ofthe first waveguide 330 may be epitaxially grown over the etch stopalignment layer 320, with the offset d₁ between the optical plan or axisof the first waveguide or waveguiding layer 330 and the top surface ofthe etch-stop alignment layer 320 epitaxially defined to preciselycomplement the second offset d2 in a second chip. An etch to locallyexpose a portion of the top surface of the etch-stop alignment layer 320may be performed in a manner similar to that described hereinabove withreference to FIGS. 11A and 11B, resulting in a structure illustrated inFIG. 11B, with the now-exposed portion of the top surface of theetch-stop layer 320 serving as the first alignment surface 351. A secondselective etch may then be performed locally through the etch-stop layer320 to expose the metal bonding pads 310, leaving substantial portionsof the etch stop layer 320 unaffected to provide the first alignmentsurface 351 as a vertical-alignment stop for the second chip, asillustrated in FIG. 13C.

FIG. 14 illustrates a hybrid semiconductor device 400 with a second chip403 flip-chip bonded to the first chip 302, in a manner similar to thatdescribed hereinabove with reference to FIGS. 12A and 12B. Similar tochip 402, in the process of assembly is flipped to be oriented with thesecond alignment surface 451 facing the first alignment surface 351 ofthe first chip 302 and preferably parallel thereto. Metal contact pad orpads 410 may be disposed upon the second alignment surface 451 in alocation suitable for mating with the first contact pads 310 of thefirst chip 301. The optical waveguiding layer or waveguide 430 of thesecond chip 403 is disposed so that its optical axis or plane isvertically offset from the alignment surface 451 by a second offset d₂,which may also be epitaxially defined to be equal in value to the firstoffset d₁ in the first chip 302, as discussed hereinabove, with anaccuracy of at least +\−200 nm, and preferably +\−100 nm. Accordingly,when the second chip 403 is placed upon the first chip 302 with theiralignment surfaces 451 and 351 abutting each other and in a firm contactacross flat portions thereof, the optical waveguides 330 and 430 of thechips are in a good vertical alignment with each other, and theiroptical planes and/or axes substantially coincidental.

With reference to FIG. 14, in the illustrated embodiment of theassembled hybrid device 400 the optical waveguides 330 and 430 arealigned in the vertical, i.e., z-axis, direction, and metal contacts 310and 410 are in electrical communication provided by a conductive bondingagent, such as solder, which bonds the chips together and provideselectrical and thermal contact between their metal pads 310 and 410. Ifrequired, precision placement equipment may be used to further align thechips in the (x,y) plane prior to the bonding so as to ensure goodoptical alignment and optical coupling between the chips' waveguides.

With reference to FIG. 15, in another embodiment a hybrid semiconductordevice 500 includes first and second chips 303 and 404, respectively,that are optically aligned and bonded together. The first and secondchips 303 and 404 may be generally similar to the first and second chips301, 402 illustrated in FIGS. 11 and 12, except that their respectivecontact pads 310 and 410 are both located with a vertical offset andaway from the respective alignment surfaces 351, 451. As in theembodiments described hereinabove, the vertical offsets of the contactpads 310, 410 from the respective alignment surfaces 351, 451 differ sothat when the alignment surfaces 351, 451 are parallel and in a directcontact with each other, the contact pads 310, 410 are separated by agap, which in the assembled device may be filled by a bonding agent, forexample solder, so as to provide mechanical bonding of the chips and, ifdesired, electrical and/or thermal contact between the chips.

The accurate vertical alignment of chips using vertical alignment stopsin the form of alignment surfaces located at precise complementaryoffsets relative to respective optical waveguides that may beepitaxially-defined in each, or at least one, of the chips in accordancewith the present disclosure may be accomplished in a variety of ways. Insome embodiments the alignment surface of one of the chips may beprovided by a top surface or surfaces of one or more pillars formed onthe main face of the chip, while the alignment surface of the other chipmay be provided in a recess or a trench that may be shaped to accept thepillar or pillars. In some embodiments one of the alignment surfaces maybe provided as a surface of a buffer, or alignment, layer that may beepitaxially grown above or below of the waveguide cladding layers. Insome embodiments the buffer layer may be selectively etched to form oneor more pillars. In some embodiments one of the alignment surfaces maybe provided by an exposed surface of a waveguiding layer or of acladding layer. Several illustrative configurations will now bedescribed with reference to exemplary embodiments wherein the first chipis a silicon-based chip and the second chip is a compound semiconductorlaser chip. In other embodiments the first and second chips beingintegrated into a single hybrid device can be chips of other types or ofthe same type, for example both may be silicon-based, with each chiphaving at least one optical waveguide that is to be aligned to anoptical waveguide of the other chip.

Referring to FIG. 16A, there is illustrated an example embodiment of ahybrid semiconductor device wherein the second chip 502, having anoptical waveguide which core is defined by a waveguiding layer 532sandwiched between cladding layers 522, 542, is flip-chip mounted on thefirst chip 501 having an optical waveguide defined by a waveguidinglayer 531 sandwiched between cladding layers. The second chip 502 haspillars 562 with epitaxially-defined flat end-faces 552 defining thealignment surface of the chip, which may be referred to herein as thesecond alignment surface 552, and which is vertically displaced from anoptical plane of the waveguiding layer 532 by an offset d₂. The secondalignment surface 552 is abutting a first alignment surface 551 of thefirst chip 501. The first alignment surface 551 of the first chip 501 isdisposed in a recess or trench formed in the top face of the first chip501 to be positioned at vertical offset d₁ from an optical axis of theoptical waveguide 531 of the first chip. The second optical offset d₂may be epitaxially-defined to precisely complement the first verticaloffset d₁, so as to result in the vertical alignment of the waveguides531, 532 of the chips.

Referring to FIG. 16B, there is illustrated another example embodimentof a hybrid semiconductor device wherein pillars 561 are provided on thetop face of the first chip 501, with flat end-faces of the pillars 561defining the first alignment surface 551 of the first chip 501. Thesecond alignment surface 552 is provided by areas of the top surface ofthe waveguiding layer 532 that are exposed in recesses or trenches 572at the top face of the second chip. In the assembled device the secondalignment surface 552 is abutting the first alignment surface 551 of thefirst chip 501, which is disposed with a precise vertical offset d₁=-d₂from an optical axis of the optical waveguide 531 of the first chip,ensuring precise vertical alignment of the waveguides 531, 532 of thechips.

Referring to FIG. 16C, there is illustrated an example embodiment of ahybrid semiconductor device wherein pillars 561, 562 are provided on thetop faces of each, with flat end-faces of the pillars 561, 562 definingthe first alignment surface 551 of the first chip 501 and the secondalignment surface of the second chip 502, respectively. Pillars 561 maybe formed in a recess defined in a top face of the first chip 501. Theflat tops of at least one of the pillars 561 and 562 may be epitaxiallydefined.

With reference to FIGS. 17A and 17B, in some embodiments one of thesemiconductor chips may be fabricated by epitaxially depositing a bufferor alignment layer 612 of a suitable material over a layer stack 672defining a planar optical waveguide in conventional photonic integratedcircuit (PIC) or optical gain chips, so as to cause the top surface ofthe buffer layer 612 to be offset from an optical plane of the secondwaveguiding layer by the desired vertical offset, e.g., d₁ or d₂. Asillustrated in FIG. 17A, this layer stack includes a waveguiding layer632 sandwiched between two cladding layers 642 and 622, with the lowercladding layer 642 typically grown over a planar substrate (not shown).The buffer or alignment layer 612 may then be deposited over the uppercladding 622, with an optional etch-stop layer therebetween. Theepitaxial growth of the layer stack 632/622/662/612 may be preciselycontrolled to provide a desired vertical offset d₁ 677 between theoptical plane or axis of the waveguiding layer 632 and the upper surface651 of the buffer layer 612, which may serve as the alignment layer forproviding a vertical alignment stop during the hybrid device assembly.As described hereinabove, this vertical offset should match acomplementary vertical alignment offset in another chip to be opticallyaligned with the chip illustrated in FIGS. 17A, B. Referring to FIG.17B, in one embodiment a recess 692 may the selectively etched in thebuffer or alignment layer 612 so as to form pillar or pillars 682 havingepitaxially-defined flat tops; the non-etched top surfaces of the pillaror pillars 682 may serve as the alignment surface 651. In someembodiments, the buffer or alignment layer 612 may itself bemulti-layer.

The entire layer stack may be fabricated by any suitable method ofthickness-controlled epitaxial growth, in which a conventional layerstack of a PIC is grown with an additional buffer or alignment layer ontop. The additional buffer or alignment layer is preferably deposited inthe same epitaxial process step as the conventional layer stack, and canbe composed of one material or of any number of different materials fromthe same material system.

The layer stack may also be fabricated in a multistep approach where theconventional layers stack 672 is formed in one process step such as butnot limited to epitaxial growth, and the additional buffer or alignmentlayer or layers are formed in a second process step using the samematerial system or a different material system. The buffer or alignmentlayer or layers may be applied using various methods, such as but notlimited to epitaxial growth, evaporative or sputter deposition, whichpreferably allow for a precise layer thickness control.

The layer and substrate materials can be chosen to allow selectiveetching that selectively removes certain material or materials and stopsat a specific epitaxially defined layer with nanometer (nm) precision.Various techniques of selective etch known in the art of semiconductorprocessing may be used, with the etching typically stopping at anetch-stop layer of a material that is known to be highly resistant tothe selected etch. The etch-stop techniques may have nm and even sub-nmheight resolution. The uniformity of the pillar structures across awafer may depend on the growth uniformity thickness across the wafer.For material selective etching, the height uniformity will be defined bythe epitaxial growth uniformity. For other etching techniques that arenot material selective, for example a dry ion etch, timing andmonitoring may be used to define the height of the pillars, and couldvary more across the wafer. Therefore layer selective etch techniquesmay be preferred. However, the methods and devices described in thepresent disclosure are not limited to using material-selective etchtechniques and etch-stop layers, as other etches may provide improvedresolution and across-wafer uniformity in time with improved processingequipment and techniques.

Referring now to FIGS. 18A-18C, in one embodiment the second chip may bea semiconductor laser chip to be optically coupled to a silicon (Si)based PIC by mounting thereon and aligning the laser waveguide with anoptical waveguide of the Si-based PIC chip. FIG. 18A illustrates by wayof example a cross-sectional view of a layer structure 712 of a buriedhetersotructure (BH) laser diode formed on an InP substrate; theillustrated BH layer and device structure is well known in the art andwill not be described herein in further detail. In accordance with anembodiment of the present disclosure, an additional buffer or alignmentlayer 722 of a precisely-defined thickness may then be deposited, forexample using epitaxial growth, over a capping layer 772 of the BH laserstructure 712, as illustrated in FIG. 18B. The epitaxial growth of thebuffer alignment layer 722 is height-controlled to provide a desiredvertical offset d₂ 777 between an optical axis of the BH laser waveguideand the top surface of the buffer layer, which may serve as thealignment surface for aligning with an optical waveguide of the PIC chipin the vertical, or z-axis, direction. The vertical position of theoptical axis may be accurately determined based on the known BH layerstructure and material properties, as will be apparent to those skilledin the art, for example by simulations or optical measurements. In someembodiments it may be approximated by a centerline of the BH waveguide.Optionally, a selective etch may then be used to remove the buffer layerin a selected area of the chip so as to form one or more pillars 732,with the top surfaces of the pillars providing the alignment surface, asillustrated in FIG. 18C. In one embodiment the selective etching isperformed so that the top surfaces of the pillars are not etched and aretherefore substantially flat, thereby providing the alignment surfacesthat are substantially flat. A protection layer 742 may be depositedover the etched are if desired.

It will be appreciated that other types of laser structures may also beused, as well as non-lasing semiconductor structures with opticalwaveguides that in some embodiments can be electrically biased, such asbut not exclusively semiconductor optical amplifier, optical modulators,optical attenuators, and LEDs. Furthermore, in some embodiments thepillars that define the alignment surfaces can be fabricated usingvarious area selective techniques such as but not limited to selectivearea epitaxial growth (SAG), evaporative or sputter deposition usinglift-off techniques, or electro-plating processes, so as to ensure thedesired accuracy of the vertical alignment offset 777, preferably +\−200nm or more preferably +\−100 nm.

In some embodiments the vertical alignment surface or surfaces of one ofthe semiconductor chips, for example the second chip to be flip-mountedon top of a first chip, may be provided in the recesses or trenchesdefined in a main face of the first chip. In some embodiments, one ofthe layers of a layer stack of the second chip may be used as analignment layer. The material of that layer may be selected to be highlyresistant to an etch technique that can remove layers grown on top ofit. In some embodiments, the waveguide layer may be of a material thatis etch-resistant and thus may serve as an etch-stop layer, for examplewith respect to a wet etch that may remove to the material of the upperand/or lower cladding layer. The intended etch stop layer can also beplaced in the upper or lower cladding layers.

With reference to FIGS. 19A and 19B, there is schematically illustrateda cross-section of a second chip 802 with a ridge waveguide defined in awaveguiding layer and electrical contacts configured for electricallybiasing the ridge waveguide before and after an etch to define verticalalignment surfaces in trenches or recesses of the structure. In aprocessing stage illustrated in FIG. 19A, the second chip 802 has awaveguiding layer 822 sandwiched between cladding layers 812 and 832.The waveguiding and top cladding layers 822, 832 have been selectivelyetched to define a ridge waveguide 872 and to allow an electricalcontact to the lower cladding layer 812, which may be electricallyconducting. Metal electrodes 842 and 862 are provided to electricallycontact the cladding layers for electrically biasing the waveguidinglayer 822 in the area of the ridge 872. In various embodiments theelectrical biasing may be used to provide optical gain, or to attenuateor modulate light propagating in the waveguide. FIG. 19B illustrates thechip 802 after performing a selective etch to remove the top cladding inselected areas except in the area of the ridge waveguide 872. Theetching step may involve a combination of a dry etch and a wet etch, ormay be performed entirely using a suitable wet etch or etches that areselective to the materials of the waveguiding and/or cladding layers. Inone embodiment, the waveguiding layer 822 itself may be selectivelyetched away at some areas of the chip to form a trench or trenches 857.An alignment surface may be provided by the exposed top surface of thewaveguiding layer 822, and/or by the exposed area or areas of the lowercladding 812 in a trench 857. The corresponding vertical alignmentoffsets, i.e., the vertical offsets of the alignment surfaces from anoptical plane or axis of the waveguiding layer, are indicated in FIG.19B as ‘d8’ and ‘d7’, respectively. By way of example, the top claddinglayer 832 may be formed of p-type InP, the waveguiding layer 822 may beformed of InGaAsP, and the bottom cladding layer 812 may be formed ofn-type InP. Wet etch solutions that are capable of selective etchingthrough InGaAsP but for which InP is an effective etch-stop are known inthe art. Similarly, wet etch solutions that are capable of selectiveetching n-type InPand p-type InP but for which InGaAsP is an etch-stopare also known in the art.

In some embodiments, at least a portion of a hybrid semiconductor deviceassembled in accordance with the description in the present disclosuremay be hermetically or semi-hermetically sealed. A hermetic seal isdefined herein as an airtight seal, while the semi-hermetic seal is aseal that protects from outside dust and contamination but may not beair-tight. However, there may be various levels of hermeticity due tothe different sizes of molecules that are air-borne and the amount oftime that it takes for one of those molecules to eventually penetratethe seal. For example molecules of hydrogen (H) are very small and canpenetrate a seal much easier than a larger molecule like methane orother hydro-carbons. Hermetic seals are typically quantified by the rateat which helium (He) will penetrate the seal, given in units of cubiccentimeters per second with a pressure differential of 1 atmosphere.This rate may be measured using a mass spectrometer at vacuum. Typicalleak rates for non-polymer hermetic interfaces are from 10⁻⁸ to 10⁻⁹cc/sec at 1 atmosphere. Example embodiments described herein relate tohermetic or semi-hermetic packaging of integrated photonics circuits atthe chip or wafer level to achieve localized hermeticity or dust andcontamination protection over sensitive areas of a single optical devicechip or a collection of optical device chips. Hermeticity orsemi-hermeticity on a local scale, that is where it is needed on a waferor chip, may be achieved in two ways—by hermetically orsemi-hermetically sealing the whole wafer or multi-chip assembly in atransparent glass or ceramic material, and by attaching a small lid overthe sensitive area on the chip to provide a locally hermetic region onthe chip. Such a lid may also serve to provide better heat sinking, andfacilitate electrical connection from the chip's top surface.

With reference to FIGS. 20 and 21, in one embodiment an entire secondchip 902 that is mounted onto a first chip 901 may be encapsulated bycovering it with a lid 999, and optionally sealing the assembly usingsuitable sealing agents, such as for example solder and/or passivationmaterials. In some embodiments the lid may remain unsealed. The smalllid 999 may be fabricated, for example, by etching a cavity in a thinpiece of ceramic, glass, or metal, or using other suitable techniquesand/or materials, to be placed over a sensitive area of the assembly. Inone embodiment, the first chip 901 may be for example a SiP chip and thesecond chip 902 may be for example a laser gain chip. The second chip902 may be optically aligned to the first chip 901 in the vertical,relative to the chip face, direction using alignment-stop surfaces 151,152, and bonded to the top of the first chip 901, for example by asolder or another bonding agent 188 at the respective contact pads 181and 182 of the chips, as illustrated in FIG. 21 and as describedhereinabove with reference to FIGS. 8-10.

The second chip 902 mounted on top of the first chip 901, for example ina recess in the top face of the first chip 901, may be covered with thelid 999, and the lid may be bonded to the first chip around itscircumference using a bonding agent 970, sealing in the entire secondchip 901 and an optical beam it may generate. In one embodiment, thehermetic seal can be created by soldering the lid 999 down to thesilicon photonics chip 901. In another embodiment, the lid 999 can bebonded to the chip 901 by another suitable material to form a hermeticor semi-hermetic seal. In some embodiments a layer of glass or ceramicmay then be evaporated or sputtered on the device to create the hermeticseal around the lid 999. In one embodiment the inside surface of the lid999 can be in mechanical, electrical and/or thermal communication withthe top of the second chip 902 to provide improved heat sinking, andoptionally to extend the electrical ground connection from the secondchip 902 to the silicon photonics chip 901.

Principles of the present disclosure have been described hereinabovewith reference to example embodiments wherein the semiconductor chips ofa hybrid optical device are assembled so that one optical waveguide of afirst chip is optically aligned to one optical waveguide of anotherchip. It will be appreciated, however, that the same principles,approaches, and techniques may be applied to chip assemblies whereineach of the chips includes multiple optical waveguides at matchingrelative locations. In particular, it will be appreciated that usingalignment surfaces at precise epitaxially defined vertical offsets froma selected optical waveguide of a waveguide array to optically alignthat waveguide to a selected optical waveguide of a matching waveguidearray of another chip will also result in other waveguides of the arraysbeing optically aligned between the chips.

Furthermore, although principles of the present disclosure have beendescribed hereinabove with reference to example embodiments wherein onesemiconductor chip having an optical waveguide is mounted on top ofanother semiconductor chip having an optical waveguide so their opticalwaveguides are aligned and optically coupled, in other embodiments twosemiconductor chips having optical waveguides may be mounted on top of acommon carrier so that their waveguides are aligned and opticallycoupled.

With reference to FIG. 22, in one embodiment an optically aligned hybridsemiconductor device 1000 may be formed of a first semiconductor chip1001 and a second semiconductor chip 1002 disposed upon a common carrier1003 having a carrier surface 1053. The first semiconductor chip 1001 isfabricated to have a first optical waveguide 1021 and a first alignmentsurface 1051 that is vertically offset from an optical axis of the firstoptical waveguide 1021 by a first offset d1 that is preferablyepitaxially defined in the first semiconductor chip 1001. The secondsemiconductor chip 1002 is fabricated to have a second optical waveguide1022 and a second alignment surface 1052 that is vertically offset fromthe optical axis of the second optical waveguide 1022 by a second offsetd2 that is epitaxially defined in the second semiconductor chip 1002.The first and second semiconductor chips 1001, 1002 are disposed uponthe common carrier 1003 with the first and second alignment surfaces1051, 1052 abutting, and in contact with, the carrier surface 1053. Thefirst and second distances d₁ and d₂ are such that when the first andsecond semiconductor chips 1001, 1002 are disposed upon the commoncarrier 1003 with the first and second alignment surfaces 1051, 1052abutting, and in contact with, the carrier surface 1053, the first andsecond waveguides 1021, 1023 are optically aligned in the verticaldirection.

In one embodiment the carrier surface 1053 upon which the chips 1001 and1002 are mounted may be substantially flat, and the first offset d1 maybe equal to the second offset d2. In one embodiment the carrier surface1053 may have a recess or a pedestal defined therein within or uponwhich one of the chips 1001, 1002 may be disposed, and the first andsecond vertical offsets d1 and d2 may differ by the height of the recessor pedestal.

In some embodiments one or both of the chips 1001, 1002 may have contactor bonding pads disposed with a vertical offset with respect to therespective alignment surfaces 1051, 1052, for example in recessesdefined in said surfaces, to be bonded to corresponding contact orbonding pads defined at the carrier surface 1053 in a manner similar tothat described hereinabove with respect to FIGS. 8, 9. In anotherembodiment, contact or bonding pads provided at the common carrier 1003may be disposed in recesses in the carrier surface 1053 to bond tocorresponding contact or bonding pads provided at one or both of thesemiconductor chips 1001, 1002.

An example of the common carrier may be an optical interposer that mayserve as a common carrier for multiple semiconductor chips. Aninterposer is known as an element that provides a high speed electricalinterface routing from one electronic device or connection to another.An interposer provides a single plane where multiple devices can connectto each other with short connection lengths for good high speed signalperformance. An optical interposer may serve as a common support and/oroptical interface for optical devices and may incorporate optical signalrouting. A desirable feature of a process of assembling optical deviceson an interposer is an ability to perform the optical alignment quicklyand accurately. Wafer and die stacking may be performed using adiffusion bonding process that maintains a high accuracy verticalplacement required for optical device assembly. This process may howeverrequire that the chips are polished flat first, and is generallycompatible with the use of flat alignment-stop surfaces defined in thechips as described herein. A different bonding solution may use solderpads that are recessed in a cavity on one of the chips that are bondedtogether with a vertical stop to provide the required vertical placementaccuracy, as described hereinabove by way of example with reference toFIGS. 8-10. Advantageously, the use of bonding pads that are verticallyoffset from flat alignment-stop surfaces in the chips doesn't requirespecial CMP planarization and is compatible with current high productionassembly processes. This bonding process could be used with otherbonding geometries as well where the chip surface is not able to beprocessed with CMP.

In one embodiment a gain chip may be directly attached to the top of thesecond photonic device chip which is then flip-chip bonded to theoptical interposer which may be Si-based and may have a cavity in it toprovide clearance for the gain chip. A gain chip is understood herein tobe a semiconductor chip incorporating an optical waveguide that may beelectrically pumped to provide optical gain. An example of a gain chipis a chip incorporating a laser diode. The assembly may be performedusing a “hard stop bonding process” described herein that is able ofachieving a high accuracy vertical bonding placement of two photonicchips. A V-groove array may be etched into the silicon interposer usingknown processes by etching along the crystal planes. A trench at theback end is used to epoxy into it an acrylate jacket of an optical fiberarray. The optical height of the fibers is controlled by the etch depthof the v-grooves and may be set, for example, to 1-10 microns above orbelow the surface, depending on the waveguide requirements of thesilicon photonics chip.

With reference to FIG. 23, there is illustrated a schematic diagram ofan example geometry for the alignment of a fiber array 1110 and a gainchip 1150 to a photonic chip assembly 1130. The photonic chip 1130 andthe gain chip 1150 may be attached via flip-chip bonding process, forexample as described hereinabove. The bonding process for this geometrymay be carried out using a diffusion bonding technique or a hard stopbonding technique described hereinabove. An assembly composing thephotonic chip 1130 and gain chip 1150 may then be attached to aninterposer 1140, again using a diffusion bonding technique or a hardstop bonding technique described hereinabove. Fibers of the opticalfiber array 1110 may be placed in grooves in the interposer 1140 thatalign the fibers with respect to the photonic chip 1130. A glass plate1120 is placed over the smaller diameter end portion of the fibers 1122in order to secure them in place, and epoxy may be used to secure theglass plate 1120 to the interposer 1140. Electrical contact between thephotonic chip 1150 and the interposer 1140 may facilitate electricalpaths to other electronic chips 1160 and 1170 that may be bonded to theinterposer 1140.

In another embodiment, the gain chip may be hard-stop bonded into acavity that is etched into the photonic chip using alignment-stopsurfaces for vertical alignment and recessed bonding pads as describedhereinabove to align the optical waveguides of both chips. The hard stopbonding may be achieved, for example, by etching shallow cavities intothe silicon photonics chip, optionally using a metal layer stop for aprecise control of the etch depth. Additional metallic layers may thenbe added at the bottom of the cavity to provide a contact point to theinterposer. Likewise, the interposer may have receiving pads that willcontact with the silicon photonics chip. Contact may be made with a thinsolder cap on one or both of the metal pads on each chip. Excess meltedsolder material will bulge or drip into the cavity instead of gettingbetween the two chips and affecting their expected placement along theZ-axis. The optical alignment of the interposer may be achieved by“flip-chip” bonding the silicon photonics chip face down onto theinterposer and aligning its optical waveguide to the v-groove positionsin the plane of the interposer surface. The gain chip maybe placeddirectly into a cavity in the second photonic chip that is then flipchip bonded to the interposer. The temperature of the two parts may thenbe raised to melt the solder material, and the two components bond downto the point where the two alignment surfaces contact each other on thehard stop. The temperature may then be lowered and the parts releasedand tested for electrical continuity. Alternatively, bonding thephotonics chip to the interposer could also be done using a diffusionbonding process. The ground side of the upside-down gain chip may beconnected to the cavity in the interposer to provide improved thermalperformance and a better ground connection. This can be achieved witheither a solder or conductive epoxy material. Finally, the fiber arrayis placed passively into position on the interposer, butted up againstthe silicon photonics chip and epoxied into position with a glass plateabove them to give them support from above.

Advantageously, the afore-described optical chip assembly techniquesthat utilize epitaxially-defined alignment-stop surfaces at preciseepitaxially-defined complimentary vertical offsets from optical axes orplanes of the chips' waveguides enable accurate passive opticalalignment of the optical waveguides of two chips in the vertical, orz-axis, direction, i.e., the direction that is normal to the plane ofthe chips. Precision-placement equipment used in modern semiconductormanufacturing may then be used to align the chips' waveguides in the(x.y) plane of the chips, optionally with the assistance of an activealignment technique when available, and/or with a possible assistance ofcamera-based imaging. Epitaxial growth techniques, such as but notlimited to vapour phase epitaxy (VPE), liquid phase epitaxy (LPE),molecular beam epitaxy (MBE), chemical vapour deposition (CVD), andmetal-organic chemical vapour deposition (MOCVD), leverage the extremelytight variance control that can be achieved in the layers of asemiconductor wafer. The deposition techniques and layer thickness canbe controlled during epitaxy to nanometer (nm) precision, and thicknessuniformity across the wafer can be held with nm tolerance. Speciallayers in the epitaxial structure, such as for example an InP based PIC,can be used to selectively stop an etch with nm precision. By utilizingthese techniques, the z-height tolerance can be reduced to nm variationswhere the height dimensions of mechanical alignment structures aredefined by combinations of the semiconductor epitaxial design and growthand the mask and etching processes. Thus utilizing epitaxially-definedalignment-stop surfaces at precise epitaxially-defined vertical offsetsfrom waveguides or waveguides' optical axes also helps in solving aproblem of tolerance buildup for aligning optical waveguide structuresbetween different photonic integrated circuits (PICs), which otherwisemay prohibit low loss passive alignment or low cost fast activealignment. The large tolerance build-ups may typically come fromthickness variations in gold and other metals when metal layers areemployed for inter-chip alignment, as well as variations in thethickness of deposited materials whose thickness cannot be tightlycontrolled.

The above-described exemplary embodiments are intended to beillustrative in all respects, rather than restrictive, of the presentinvention. Indeed, various other embodiments and modifications to thepresent disclosure, in addition to those described herein, will beapparent to those of ordinary skill in the art from the foregoingdescription and accompanying drawings. Thus, such other embodiments andmodifications are intended to fall within the scope of the presentdisclosure.

For example, it will be appreciated that although example embodimentsdescribed hereinabove relate to optical alignment of two chips to form asingle hybrid optical device, in other embodiments substantially thesame method and techniques may be used for wafer-to wafer and chip-towafer alignment, for example to integrate and optically align two wafersso that a plurality of optical waveguides defined in a first wafer isoptically coupled to a plurality of optical waveguides defined in asecond wafer, or to mount one or more optical chips on a wafer whileoptically aligning the chips to respective waveguides defined in thewafer. Thus, the term “chip” as used here should be understood toencompass a wafer or any portion of a wafer.

Furthermore, it will be appreciated that semiconductor materials otherthan silicon, including but not limited to compound semiconductormaterials such as GaAs, InP, and their alloys, as well as variousdielectric materials such as but not limited to glasses, ceramics andpolymers, may be used to fabricate the hybrid optically-aligned devicewith optically aligned waveguides on adjacent chips. For example, atleast one of the first or second chips 101, 102 described hereinabovemay be a non-semiconductor chip, including a non-semiconductor wafer.

Further for example, although in the example embodiments describedhereinabove the optical waveguides of the first and second chips areshown to be butt-coupled, i.e., disposed so that light from an end-faceof one waveguide is coupled directly into the end-face of anotherwaveguide, other embodiments may include coupling optics disposedbetween the end-faces of the waveguides, such as but not limited to aball lens or an aspheric lens.

Furthermore although in the example embodiments described herein theoptical waveguides are aligned for end-face optical coupling whereintheir optical axes and/or places are substantially coincidental, thevertical alignment techniques using alignment-stop surfaces at precisevertical offsets from the waveguide's axes of the present disclosure mayalso be adopted to align the optical chips for transverse transfercoupling of their waveguides, in which the waveguides extend alongsideeach other along a common length thereof. Furthermore, it will beappreciated that the optical waveguides of one or each of the chipsbeing optically aligned may terminate at an angle and/or have angledend-faces, for example to reduce back-reflections, as may belithographically defined in one or both of the chips. Furthermore, insome embodiments the waveguides may have end-faces that are angled withrespect to the vertical direction that is normal to the plane of thechip, wafer, or waveguide. In such embodiments the first and secondoffsets, which define the vertical location of the alignment layersrelative to the optical axes of the waveguides, may differ in value andcomplement each other in a manner that leads to a small vertical offsetbetween the optical waveguides of the first and second chips in theassembled hybrid device that is selected so as to maximize, or at leastincrease, optical coupling between the waveguides when the end-faces ofthe waveguides are angled and non-vertical. Furthermore, in someembodiments at least one of the first and second offsets may be zero.

Furthermore, although in some of the example embodiments describedhereinabove the z-axis position of the alignment surfaces of both thecarrier semiconductor chip and the semiconductor chip to be mounted ontop of the carrier chip in their respective layer stacks are defined byepitaxy so as to ensure that their offsets from the respective opticalwaveguides or optical axes are accurately defined by epitaxy, it may besufficient that only one of the alignment surfaces is defined by epitaxyso that its offset from the optical axis of its respective waveguideprecisely complements the alignment surface—optical axis offset of theother chip. Furthermore, in other embodiments the alignment surfaces maybe formed by high-precision techniques other than epitaxy, such as forexample precision etching, non-epitaxial deposition, metallization, CMP,and other. Preferably, such techniques should provide the desiredaccuracy of the first and second offsets that define the alignmentsurfaces' vertical positions relative to the respective opticalwaveguides, preferably within +\−200 nm. In some embodiments, thealignment surface offsets may be allowed to deviate within +\−1 micronor more from their target values, for example when the opticalwaveguides are provided with well-engineered mode expanders.

Furthermore, although the theoretical description that may be givenherein is thought to be correct, the operation of the devices describedand claimed herein does not depend upon the accuracy or validity of thetheoretical description. That is, later theoretical developments thatmay explain the observed results on a basis different from the theorypresented herein will not detract from the inventions described herein.

Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes.

Thus the present invention is capable of many variations in detailedimplementation that can be derived from the description contained hereinby a person skilled in the art. All such variations and modificationsare considered to be within the scope and spirit of the presentinvention as defined by the following claims.

What is claimed is:
 1. A method of fabricating an optically alignedhybrid semiconductor device, the method comprising: a) providing a firstsemiconductor chip comprising a first alignment surface, a first bondingpad, and a first optical waveguide, wherein the first alignment surfaceis positioned with a first offset from an optical axis of the firstoptical waveguide; b) providing a second semiconductor chip comprising asecond alignment surface, a second bonding pad, and a second opticalwaveguide, wherein the second alignment surface is positioned with asecond offset from an optical axis of the second optical waveguide,wherein one of the first and second offsets is epitaxially-defined inthe respective first or second semiconductor chip to complement theother of the first and second offsets so that when the first and secondalignment surfaces are in contact abutting each other, the first andsecond waveguides are aligned in a direction normal to the first andsecond alignment surfaces; c) aligning first and second semiconductorchips with the first and second alignment surfaces facing each other;and, d) bringing the first and second semiconductor chips together untilthe first and second alignment surfaces come to a stop against eachother, with the first and second alignment surfaces being in directcontact with each other and the first and second optical waveguidesbeing optically coupled, wherein providing at least one of the first andsecond semiconductor chips includes: growing a stack of epitaxial layerscomprising the first or second optical waveguide sandwiched between twocladding layers, and forming one of the first alignment surface or thesecond alignment surface by selectively etching the stack of epitaxiallayers using a layer-selective etch so as to expose at least an area ofan epitaxy-defined layer surface of one of the epitaxial layers of thestack; wherein one of the first and second bonding pads is offsetrelative to one of the first or second alignment surfaces in therespective first or second semiconductor chips so that when the firstand second alignment surfaces are brought in contact with each other instep (d), there exists a gap G between the first and second bondingpads.
 2. The method of claim 1, further comprising disposing a compliantbonding agent upon at least one of the first and second bonding padsprior to (d), and causing the compliant bonding agent to connect thebonding pads in step (d).
 3. The method of claim 2 wherein the bondingpads are electrically conductive, wherein the bonding agent compriseselectrically conducting solder, and wherein the method includes heatingthe electrically conducting solder to a near-melting temperature.
 4. Anoptically aligned hybrid semiconductor device comprising: a firstsemiconductor chip comprising a first alignment surface and a firstoptical waveguide having an optical axis, wherein the first alignmentsurface is positioned with a first offset from the optical axis of thefirst optical waveguide, wherein the first semiconductor chip comprisesa substrate; and, a second semiconductor chip comprising a secondalignment surface and a second optical waveguide having an optical axis,wherein the second alignment surface is positioned with a second offsetfrom the optical axis of the second optical waveguide, wherein thesecond offset is complementary to the first offset, wherein the secondsemiconductor chip comprises a compound semiconductor substrate and astack of epitaxial layers grown thereupon so as to define the secondalignment surface at an exposed surface of one of the epitaxial layersof the stack; and wherein the second semiconductor chip is disposed uponthe first semiconductor chip so that the second alignment surfacedirectly abuts the first alignment surface in a contact therewith, andthe first and second optical waveguides are aligned and opticallycoupled; wherein each of the first and second semiconductor chipsfurther comprises a bonding pad, said bonding pads disposed to face eachother with a gap therebetween, wherein the gap comprises a bonding agentfor bonding the second chip to the first chip, and wherein one of thebonding pads is disposed upon a surface of the second semiconductor chipthat is offset from the second alignment surface.
 5. A method offabricating an optically aligned hybrid semiconductor device, the methodcomprising: a) obtaining a first semiconductor chip comprising a firstalignment surface and a first optical waveguide formed upon a substrate,wherein the first alignment surface is positioned with a first offsetfrom an optical axis of the first optical waveguide; b) obtaining asecond semiconductor chip comprising a second alignment surface and asecond optical waveguide formed upon a compound semiconductor substrate,wherein the second alignment surface is positioned with a second offsetfrom an optical axis of the second optical waveguide, wherein the firstand second offsets are complementary so that when the first and secondalignment surfaces are in contact with each other, the first and secondwaveguides extend in a common plane; wherein obtaining the secondsemiconductor chip includes using thickness-controlled epitaxy to grow afirst stack of epitaxial layers upon the compound semiconductorsubstrate so as to define a position of the second alignment surface inthe first stack of epitaxial layers at a layer surface of a topepitaxial layer thereof; c) aligning first and second semiconductorchips with the first and second alignment surfaces facing each other;and, d) bringing the first and second semiconductor chips together untilthe first and second alignment surfaces come to a stop against eachother, with the first and second alignment surfaces being in directcontact abutting each other and the first and second optical waveguidesbeing optically coupled; providing each of the first and secondsemiconductor chips with a bonding pad that is electrically conductingand is disposed upon an electrically conducting surface of therespective chip, the bonding pads positioned to face each other with agap therebetween when the first and second semiconductor chips broughttogether in step (d) and the first and second alignment surfaces are indirect contact abutting each other; depositing a compliant electricallyconducting bonding agent upon at least one of the bonding pads to atotal height exceeding the gap; wherein d) comprises: bringing the firstand second semiconductor chips together to first cause the compliantelectrically conducting bonding agent to touch the bonding pads of boththe first and second semiconductor chip; bringing the first and secondsemiconductor chips further together until the first and secondalignment surfaces come to a stop against each other; and, causing thecompliant electrically conducting agent to solidify, thereby providingelectrical and mechanical bonding between the first and secondsemiconductor chips.
 6. The method of claim 5, wherein step a)comprises: growing a second stack of epitaxial layers including thefirst optical waveguide, a cladding layer, and an etch-stop layer with atop layer surface of the etch-stop layer disposed at the first offsetfrom an optical plane of the first optical waveguide, and selectivelyetching the second stack of epitaxial layers up to the etch stop layerso as to expose at least an area thereof to form the first alignmentsurface.
 7. The method of claim 1, wherein step a) comprises: growing afirst stack of epitaxial layers including the first optical waveguide, acladding layer, and an etch-stop layer with a top layer surface of theetch-stop layer disposed at the first offset from an optical plane ofthe first optical waveguide, and selectively etching the first stack ofepitaxial layers up to the etch stop layer so as to expose at least anarea thereof to form the first alignment surface.
 8. The method of claim1, wherein step b) comprises using thickness-controlled epitaxy to growa second stack of epitaxial layers upon a compound semiconductorsubstrate so as to define a position of the second alignment surface inthe second stack of epitaxial layers at a layer surface of a topepitaxial layer thereof.
 9. The device of claim 4, wherein the firstsemiconductor chip includes a cladding layer, and an etch-stop layerwith a top layer surface of the etch-stop layer disposed at the firstoffset from an optical plane of the first optical waveguide, and whereinthe first alignment surface comprises an exposed area of the etch stoplayer.
 10. The device of claim 4, wherein the second semiconductor chipincludes a second stack of epitaxial layers upon the substrate; andwherein the second alignment surface comprises a layer surface of a topepitaxial layer thereof.